SEUS PTGI is a beast. It uses advanced OpenGL features (like ARB_bindless_texture ) that were historically only supported by OptiFine’s custom rendering pipeline. Iris uses a different method (Rendering Engine Rewrite).
If your resource pack relies on specular maps, normal maps, or emissive textures designed for SEUS (like realistic stone bricks that get wet in the rain), The blocks will look flat. Vanilla PBR support is getting there, but advanced PTGI-specific PBR is lagging. seus ptgi iris support